1. Technical Field
The present invention relates to the field of DC motor technology. It relates in particular to a high-power DC motor, especially for model racing vehicles.
2. Prior Art
Small, extremely high-power DC motors which rotate at a high speed are used nowadays for driving battery-powered model racing cars, model boats or model aircraft. Examples of such high-power DC motors for model racing cars are the RC2140 or RC2141 types from the American company Trinity Products, Inc. Edison, N.J. (USA), or the “Chrome Touring” series of motors from the Swiss company Team Orion Europe SA, 1233 Bernex, Switzerland. FIGS. 1A and 1B show an example of the design of such DC motors from the prior art in simplified form and in the form of a detail, with the figure element 1A showing a longitudinal section through the motor and the figure element 1B showing a plan view in the axial direction of the motor head. The DC motor 10 shown in FIGS. 1A and 1B has a housing 16 with magnets 17 (the fixed part) which are arranged on the inside, and in which a rotor 11 (the rotating part of the motor) is mounted such that it can rotate about an axis 29. The rotor 11 comprises a central shaft 15, an armature 12 with appropriate windings 13 and a commutator 14 (for the sake of simplicity, only one of the windings is shown).
The armature 12 is composed, in a known manner, of a material with high magnetic permeability. An electrical current flows through the windings 13, producing a magnetic field when the DC motor 10 is supplied with current, and with this magnetic field interacting with the magnets (permanent magnets) 17 on the housing 16. The commutator 14 is composed of individual electrically conductive commutator segments 20, normally made of metal, which are isolated from one another and are arranged on a cylindrical casing surface about the axis 29. The commutator segments 20 are electrically connected to the windings 13 via winding connections 18 in a predetermined manner. The commutator controls the time duration and direction of the current flow through the windings 13, such that the attraction and repulsion forces cause the rotor 11 to rotate depending on the magnetic field direction in the armature 12 and on the polarity of the magnets 17.
The housing 16 of the DC motor 10 is provided at both ends with sliding bearings, ball bearings 21 or similar bearings, which guide the central shaft 15 of the rotor 11. Furthermore, brush boxes 24 which extend in the radial direction and in which brushes 22, 23 are guided are arranged on opposite sides in the motor head 19 in which the commutator 14 is accommodated. Brushes 22, 23 may be composed of carbon. However, they may also be composed of a material other than carbon, in particular and also including a material mixture, for example of carbon, graphite, copper, silver or the like. The inner ends of the brushes 22, 23 are seated on the commutator 14 and in this way guide the current from the rigid part of the motor via the commutator 14 to the windings 13 of the rotor 11. The brushes 22, 23 are pressed against the commutator 14 by pressure springs 27, and are electrically conductively connected via current-carrying braids 25 to electrical connecting poles 26.
During operation of the DC motor 10, centrifugal forces act on the rotor and generally lead to the central shaft 15 no longer rotating centrally. The radial gap between the central shaft 15 of the rotor 11 and the bearings, in particular the gap in the ball bearings 21 allows the axis of the rotor 11 to be shifted radially and the rotor 11 not to rotate roundly. When sliding bearings are used, the gap increases as the sliding bearings wear.
Dynamic balancing of the rotor 11 only partially improves the situation. The shifting of the centre can also be caused by unequal magnetic forces. The drive force of the motor when the motor is installed is transmitted to other rotating parts. When power is drawn, a lever effect is produced which forces the rotor 11 against the bearing walls. The lever effect can become even greater during acceleration and braking. These forces vary over the course of operation of the motor and force the rotor 11 away from the ideal centre.
The uncentred rotation of the shaft 15 of the rotor 11 results in the commutator 14 not rotating centrally either, that is to say not running roundly. The brushes 22, 23 are forced against the commutator segments 20 by pressure springs 27. Thus, when the rotor 11 is not running roundly, the brushes 22, 23 have to follow the backward and forward movement of the commutator 14. At low rotation speeds, the brushes 22, 23 can follow this by moving backward and forward in the movement direction 28 in their brush boxes 24 (FIG. 1B), so that the contact between the brushes 22, 23 and the commutator segments 20 is always good.
However, as soon as the rotation speed rises, the brushes 22, 23 are knocked away in the movement direction 28 by the commutator 14, lift off the commutator 14 and do not make contact with the next commutator segment 20 again until later. The electrical contact is made with a time delay, and the power of the motor falls.
If the brushes 22, 23 do not slide smoothly from one commutator segment 20 to the next, the electrical contact is interrupted at times. The interruption and the renewed making of the contact lead to sparks being formed between the brushes 22, 23 and the commutator segments 20. These sparks burn the commutator segments 20. The commutator segments 20 are damaged, and the quality of the current transmission suffers and falls. The commutator 14 is worn away prematurely, and the power of the motor decreases permanently. The sparks and the damaged commutator segments 20 also increase the wear rate of the brushes 22, 23, and overheat them.
Sparks also form a heat source, so that the commutator segments are deformed and are worn away irregularly. This adversely affects the life of the motor, and reduces its power.
The sparks also cause radio interference and can interfere with remote control receiving systems, or even make them unusable, when motors such as these are operated in the vicinity of radio receivers.
It is already known from WO-A1-01/69760 or JP-A-07-194067 for the backward and forward movements of the brushes to be damped by using a damping substance (for example a thixotropic material) to guide the springs, which also move. It is also known for the movement of the brushes to be braked by mechanical friction, by a leaf spring pressing against the side of the brushes. Both the damping and the braking are symmetrical, that is to say they are of equal intensity in both movement directions of the brushes. Both the movement of the brushes away from the commutator and the movement of the brushes in the direction of the commutator are thus damped or braked. This damping (braking) thus delays the time at which contact is next made between the brushes and the commutator segments. No improvements can be achieved in this way. What is obtained on the one hand (when the brushes are lifting off the commutator) is made worse on the other hand (when the brushes are moving towards the commutator).
Furthermore, the current is normally passed via a current transmission braid (25 in FIG. 1B), which is fixed in the brush, to the brushes, and then through the brush to the commutator. Overall, this forms an electrical resistance. If this electrical resistance could be reduced, then the motor would produce more power The known solutions for damping and braking the backward and forward movement of the brushes do not allow any improvements whatsoever with regard to reducing the magnitude of the resistance.
U.S. Pat. No. 2,991,379 and the parallel DE-B-1 122 625 describe a brush holder for a servomotor, in which the longitudinal axes of the brushes in the brush boxes form an angle of about 45° with the motor axis. In order to avoid increased wear on the brushes and to achieve increased no-load rotation speeds without having to increase the brush pressure, it is proposed that the brushes be guided better in the brush boxes. For this purpose, according to one embodiment (FIGS. 1A and 1B), the brush boxes are designed to have a polygonal (square, hexagonal) cross section, such that the motor axis lies on the plane which passes through two opposite edges of the brush boxes. The brushes have a corresponding cross-sectional shape and are guided by two opposite longitudinal edges in the grooves formed by the corners in the brush holders. The use of the edges of the brushes for guidance results in a greatly reduced contact area between the brushes and the brush boxes, which on the one hand prevents effective friction damping of the brush movement, and on the other hand makes it more difficult to supply current to the brush tip via the brush box.
In a second embodiment of U.S. Pat. No. 2,991,379 (FIG. 2), the brush is guided in a guide element which is open on one side, is in the form of a rail and has a cross section in the form of a right angle. The spring which provides the brush with pressure is inclined with respect to the longitudinal axis of the guide element such that the brush is forced into the guide element, which is in the form of a groove, not only by the commutator forces but also by the opposing spring forces. This admittedly results in increased friction damping, but the friction damping is independent of the movement direction of the brushes.
U.S. Pat. No. 5,696,418 describes an electrical commutator machine in which the two brushes are arranged offset from the radial direction with respect to the machine axis. A special mounting is proposed for the brush boxes on a plate which surrounds the machine axis and is at right angles to the machine axis. The brush boxes, which are bent from sheet metal, have a rectangular cross section and are inserted into corresponding slots in the plate, and are secured, by means of lugs which project at right angles on the lower face. A spring tongue is formed (FIGS. 5, 6) in one side wall of the brush box, presses on the one side against the brush which is located in the brush box, and thus presses this against the opposite wall of the brush box. This pressure mechanism not only results in the brush being fixed in the box but also results in uniform friction on the opposite wall, which is independent of the movement direction of the brush.
Finally FR-A-2 723 481 discloses a brush holder for reversible commutator machines, in which two pairs of brush boxes are provided, which are each arranged offset from the radial direction with respect to the motor axis. The brush holder is produced from a plastic, as an injection-moulded part. The individual brush boxes have a square cross section and are provided at the corners with rails (42) which project inwards and in which the brushes are guided by their edges. The use of plastic for production of the holder and the specific nature of the edge bearing mean that the holder is not suitable for use in high-power motors, because the heat which is produced on the commutator cannot be effectively dissipated in the brushes, because the current cannot be passed directly into the front part of the brush and because this results in only a small amount of friction damping for the moving brush.
The solution to the problems which occur with the brushes in high-power DC motors for model racing cars is made very much worse by the fact that the existing regulations (for racing operation) mean that the motors are on the one hand subject to major restrictions with regard to the geometry (external dimensions) and electrical and mechanical design, and on the other hand have to achieve extreme power levels (rotation speed, acceleration time, torque, etc) at least over time periods of several minutes. The motors can in this case reach rotation speeds of up to 60 000 rpm, and draw a current of up to 120 A from the battery or rechargeable battery set, which is likewise subject to restrictions. The motor heads in which the brushes are arranged may in this case be heated up to 100° C. In the extreme, the brushes last for only a single race, which is only 5.5 minutes.
Racing with model racing cars which are driven electrically is subject to internationally applicable laws, the so-called ROAR (Remotely Operated Auto Racers) Rules, which specify, inter alia, the restrictions and boundary conditions applicable to electrical drives. The ROAR rules state that the motor must not have an external diameter of more than 36.02 mm and must not have a length of more than 53 mm, measured from the mounting plate at one end to the outermost point of the motor head. The diameter of the central shaft must be ⅛ inch. Only ceramic magnets are allowed. The commutator may have only three sectors. In the same way, a specific number of turns are specified. Based on specific basic motor types, technical changes may be made to the motors within the rules, with such changes having a different scope depending on the motor class. In this case, a distinction is drawn between so-called stock motors, rebuildable stock motors, and modified motors. The most extensive changes which also include, in particular, changes to the brushes and to the internal design of the motor, may be made in the last-mentioned motor class. However, owing to the extreme space restriction, the brushes can be modified only with major difficulties.